the do-me decimal system
by Le Penguin
Summary: There are books in the library, but also privacy, and Eren likes his odds. (Eren/Armin)
1. they get interrupted here

Eren walked into the library to discover two very important things.

One, Armin was there.

Two, no one else was.

One plus two equaled a great place to have sex, in Eren's very expert opinion as a professional horny sixteen-year-old. Armin seemed less convinced of Eren's arithmetic when he sidled up behind him to grab his ass.

"Eren!" he yelped. "We're in the—"

"Library. Yeah." Eren's hands snuck around to Armin's front, to smooth down his hip and to his thighs. "No one else is, though."

Armin gave another murmur of protest, this one not as firm as the last, especially when Eren's lips came down to brush against the shell of his ear. He grumbled and leaned a bit heavier against the bookshelf when Eren tilted his hips forward to rub on him.

"You're ridiculous." Armin gave a little _ah_ as Eren's lips and teeth found his neck. "Did you…did you at least lock the door?"

Eren's hands wandered up the insides of Armin's thighs, groped upward to rub between his legs. He let out a groan as the motion encouraged Armin to rub backward against his dick. Fuck, was Armin's ass amazing, and talented in so many different ways.

"I…think I shut it—"

"You didn't," came Mikasa's flat voice.

Armin stiffened and jumped as if he'd been scalded, and cracked the back of his head against Eren's nose. Eren reeled backward into the facing bookshelf. He growled and pressed his hand to his aching face, glowering at Mikasa through the tears of pain and thwarted dick-on-ass action.

"You didn't have to stop," Mikasa said calmly. "I was just informing you."

"Yeah, thanks a bunch." Eren winced and wiped the blood from his nose off on his hand. Armin made a small, concerned noise, and approached with a handkerchief from his pocket.

"Should I carry you to the infirmary now or wait until you've completed copulating?"

Eren swore at the stab of pain from Armin dabbing at his nose, as well as Mikasa's flagrant disrespect for his ambitions of dry-humping Armin against a bookshelf. Armin gave her a small, red-faced smile from underneath his bangs.

"I—I think the moment has passed," he explained.

Mikasa's head tilted to the side. "Could I be of any assistance? I could bring you a snack from the mess hall to set the mood."

"FUCKING LEAVE US ALONE," Eren half-screamed, half-cried.

Mikasa did not, and hauled a struggling, swearing Eren to the infirmary by his ankles.


	2. this is the one where they don't

Eren walked into the library to discover two very important things.

One, Armin was there.

Two, no one else was.

One plus two equaled a great place to have sex, in Eren's very expert opinion as a professional horny sixteen-year-old. Armin seemed less convinced of Eren's arithmetic when he sidled up behind him to grab his ass.

"Eren!" he yelped. "We're in the—"

"Library. Yeah." Eren's hands snuck around to Armin's front, to smooth down his hip and to his thighs. "No one else is, though."

Armin gave another murmur of protest, this one not as firm as the last, especially when Eren's lips came down to brush against the shell of his ear. He grumbled and leaned a bit heavier against the bookshelf when Eren tilted his hips forward to rub on him.

"You're ridiculous." Armin gave a little ah as Eren's lips and teeth found his neck. "Did you…did you at least lock the door?"

Eren's hands wandered up the insides of Armin's thighs, groped upward to rub between his legs. He let out a groan as the motion encouraged Armin to rub backward against his dick. Fuck, was Armin's ass amazing, and talented in so many different ways.

"I…think I shut it—"

"'Think'?" Armin's head craned over his shoulder to try and catch a glimpse. "Eren, if we…if we get caught…"

Armin trailed off into a breathy sigh as Eren's hand rubbed heavier between his legs. His thighs twitched and squeezed around Eren's fingers. Eren nudged his feet apart wider, bent him to lean fully against the bookshelf while Eren's dick pressed and rubbed against him. The firm, soft swell of Armin's ass was amazing, better than amazing, but fuck, Eren needed more. He groaned and gave a wet kiss to the back of Armin's neck as his hands fumbled at his belt.

"I'm gonna unzip," Eren panted against his skin. "Armin…"

Armin gave a frantic little nod of agreement, and undid his belt and zipper with shaking hands. He gave another glance over his shoulder at the door, before shimmying his pants down his hips just enough to free his cock, and give Eren a perfect eyeful of his backside. Eren groaned and bent Armin over again, his dick rubbing hard and hot and leaking into the cleft of his ass. His hand scrambled around to wrap around Armin's dick, and Eren felt a shivering thrill go down his spine at the way every jerk of his hips made Armin fuck forward into his fist.

Maybe it was the thrill of getting caught, or maybe Armin's ass was just that amazing, but neither lasted long. Eren collapsed, panting, against him. Armin's hand reached back to stroke at his hair.

"If you were curious," came Mikasa's voice from the library's entrance. "You didn't shut the door."


End file.
